Ruminal in vitro degradation of protein in tannin-free and tannin-containing forage legume species

Protein in forages often is poorly utilized by ruminants because of extensive degradation in the rumen. Our objectives were to assess the variation in ruminal protein degradability among forage legumes and to determine whether degradability was reduced by tannins. Legumes representing a range of species adapted to temperate regions were harvested on 29 Aug. 1989 and 31 Aug. 1990 from triplicate plots, then lyophilized, ground, and analyzed for tannins by radial diffusion. Ruminal protein degradation rates and escapes, estimated assuming a passage rate of 0.06 h(-1), were determined both years by an in vitro limited substrate inhibitor procedure, and on samples from 1990 by a Michaelis-Menten method. Protein degradation rates and escapes (limited substrate method) in 1989 ranged from 0.27 h(-1) and 18% for white clover (Trifolium repens L.) to 0.001 h(-1) and 96 and 97% for two cultivars of sericea lespedeza [Lespedeza cuneata (Dum.-Cours.) G. Don]. In 1990, rates and escapes ranged from 0.21 h(-1) and 21% for alfalfa (Medicago sativa L.) to virtually 0 and 93 to 97% for two cultivars of sainfoin (Onobrychis viciifolia Stop.) and three of sericea lespedeza. More rapid degradation rates were obtained with the Michaelis-Menten approach for the most slowly degraded entries. Differences in degradation rate and ruminal escape were proportional to tannin concentration. However, red clover (Trifolium pratense L.), which does not contain tannins, had a protein degradability comparable with forages with low levels of tannin. These data suggest that differences in ruminal protein degradation among forage legumes are only partly explained by the presence of tannins.